As a GP the subject of GANFYDs ('Get a note from you doctor') is a source of huge frustration for the profession. The consensus among GPs is that we are not trained or indemnified in aviation medicine and should not provide 'fit to fly' notes for any medical condition or pregnancy. That's not to say that notes can't be provided that simply state how many weeks pregnant a woman is, but it's a waste of NHS and patients' time.
Airlines and their regulatory bodies have their own guidance on when pregnant women can fly and it is beyond pointless to ask for a doctor's note if the number of weeks of gestation can be proven with scan reports or other evidence.
(I know this doesn't necessarily help you)